This bumps the version of our dependency, shelljs-release. I just cut
the 3.0 release for shelljs-release, which contains support for the
`--otp` flag, as well as the significant refactoring work we put in.

This changes appveyor from using the latest npm version to only using
the preinstalled npm version. This is safer, because it's guaranteed to
be a compatible version.

Fixes #844

Knowing the `npm` version was a huge help to debugging issue #844. For
some reason, we output this on appveyor but not on Travis. This PR fixes
that.

* Prevent require-ing bin/shjs

* Move require guard up, and improve function name

* Move require up and clarify comment

No change to logic.

This reduces install spam by bumping codecov to avoid depending on a
deprecated graceful-fs version, as well as adding a package-lock.json.

This fixes a conflict between mocks.init() and
utils.skipOnWin/skipOnUnix. mocks.init() mocks out process.stderr.write,
which utils.js implicitly depends on.

Instead, preserve stderr.write in a local variable to avoid polluting
mocked stdio and to correctly output warning messages.

Fixes #862
Test: locally apply mocks.init() inside test/which.js

This is a redo of PR #555.

This rebases, cleans up a test, and fixes a bug (the original PR uses `fs.chown()` instead of `fs.chownSync()`).

Fixes #98

No change to logic.

This renames some tests to be a bit more readable.

On Unix, this only matches files with the exec bit set. On Windows, this only
matches files which are readable (since Windows has different rules for
execution).

Fixes #657.

grep includes the i flag to ignored upper/lower case differences

No change to logic.

We don't really use the gitter chatroom right now, so we shouldn't
direct users to chat there. It's best to have issues reported directly
on Github.

This PR removes the gitter badge from the README.

No change to logic.

This changes appveyor to run the entire test matrix, even if a failure
occurs early on. This is helpful for debugging.

* chore(npm): add ci-or-install script

This adds a script which checks the npm version and runs either `npm ci`
or `npm install` based on support. This is primarily to work around an
issue where `npm install` modifies `package-lock.json` for newer npm
versions.

A side benefit is that `npm ci` is slightly faster than `npm install`.

Fixes #893

Unconditionally apply `silent: true` when calling `common.error()` from
`exec()`. This is because errors are already printed to stderr, or are
intentionally silenced by `shell.config.silent`.

Based on #861 

Fixes #851

Apparently, travis hasn't been uploading any coverage data this whole
time. This fixes the command (copied from appveyor).

Previously, the cached `tempdir` value was stored in `common.state`.
Unlike the other `common.state` values, this isn't immediately useful to
other commands (they can just call the tempdir API). So, this moves the
cached value into `tempdir.js`.

This also adds a unit test for the caching behavior, and exposes
test-only helpers to verify this behavior.

Finally, this adds a note to `common.state` that values should generally
be considered read-only, since this can be important for customized
behavior. Although, I recognize our code base has one exception to this
rule (`echo()`), we should strive to maintain this.

Fixes #902
Test: Added a unit test.

As discussed as an aside in #900, add test case with an empty file.