Fix double-loading of constants in development

When developing Overcommit locally, if you ran `overcommit --run` you
would encounter an issue where you would see a bunch of warnings
displayed related to already-initialized constants, e.g.

    .../projects/overcommit/lib/overcommit/constants.rb:6:
      warning: already initialized constant Overcommit::HOME
    .../.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/overcommit-0.39.1/lib/overcommit/constants.rb:5:
      warning: previous definition of HOME was here

This was ultimately caused by the fact that in the Gem specification
(`overcommit.gemspec`) we were modifying the `$LOAD_PATH` to include the
local repository's `lib` directory. This is problematic because if the
user had run `gem install overcommit` previously, that version of
Overcommit would already be in the `$LOAD_PATH`, e.g.

    > $LOAD_PATH.grep(/overcommit/)
    .../.rbenv/versions/2.4.1/lib/ruby/gems/2.4.0/gems/overcommit-0.39.1/lib
    .../projects/overcommit/lib

Notice that the local development directory under `projects` comes
_after_ the version of Overcommit installed via `gem install ...`.

A simple workaround is to not modify the `$LOAD_PATH` and to use
`require_relative` instead, thus loading the constants directly when
initializing the Gem specification.

Author: sds

overcommit.gemspec

@@ -1,6 +1,5 @@
-$LOAD_PATH << File.expand_path('../lib', __FILE__)
-require 'overcommit/constants'
-require 'overcommit/version'
+require_relative './lib/overcommit/constants'
+require_relative './lib/overcommit/version'
 
 Gem::Specification.new do |s|
   s.name                  = 'overcommit'