Drop support for Ruby 1.8.7

The time has finally come. While Ruby 1.8.7 technically EOLed a while
ago, we were continuing to support it so users running older versions of
OS X operating systems (and many Linux distributions) would be able to
install it without doing anything.

This was a pain, but not too much hassle. However, a dependency for our
build infrastructure, RuboCop, depends on a gem that does not support
Ruby 1.8.7. It's not worth it for us to jump through hoops to continue
to support 1.8.7, so let's call it a good run and end it now.

At some point we'll follow up with converting all the 1.8 hashes to 1.9
syntax, and removing any other 1.8.7 hacks.

Change-Id: I9a048062bd905c685b8e5a633ca44af7cd1a7879
Reviewed-on: http://gerrit.causes.com/40470
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

CHANGELOG.md

@@ -9,6 +9,7 @@
 * Fix performance regression where running Overcommit in a repository
   with a lot of files would be very slow
 * Wildcards in include/exclude globs now match files beginning with `.`
+* Drop support for Ruby 1.8.7
 
 ## 0.13.0
 

README.md

@@ -32,7 +32,6 @@ about Overcommit on our [engineering blog](http://causes.github.io).
 
 The following Ruby versions are supported:
 
-* 1.8.7
 * 1.9.3
 * 2.0.0
 * 2.1.x

overcommit.gemspec

@@ -24,10 +24,9 @@ Gem::Specification.new do |s|
                             Dir['libexec/**/*'] +
                             Dir['template-dir/**/*']
 
-  s.required_ruby_version = '>= 1.8.7'
+  s.required_ruby_version = '>= 1.9.3'
 
   s.add_dependency             'childprocess', '>= 0.5.1'
-  s.add_dependency             'json', '>= 1.8' # For 1.8.7. support only
 
   s.add_development_dependency 'image_optim', '~> 0.13.0'
   s.add_development_dependency 'rspec', '~> 3.0'