Remove RUBYOPT env variable hack from execute

This was included a long time ago to make developing hooks easier.
However, it's hacky, but more importantly it interferes with Travis CI
runs since it kills the existing bundler context, resulting in issues
like:

    /home/travis/.rvm/rubies/ruby-1.9.3-p551/lib/ruby/site_ruby/
    1.9.1/rubygems/dependency.rb:315:in `to_specs': Could not find
    'bundler' (>= 0) among 42 total gem(s) (Gem::LoadError)

Remove it to fix this issue.

Change-Id: Id1b8181def00026ee0b6857614c31d576f9243fd
Reviewed-on: http://gerrit.causes.com/46974
Tested-by: jenkins <jenkins@brigade.com>
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>

Author: sds

lib/overcommit/utils.rb

@@ -101,19 +101,20 @@ def in_path?(cmd)
         false
       end
 
-      # Wrap external subshell calls. This is necessary in order to allow
-      # Overcommit to call other Ruby executables without requiring that they be
-      # specified in Overcommit's Gemfile--a nasty consequence of using
-      # `bundle exec overcommit` while developing locally.
+      # Wrap external subshell calls.
+      #
+      # This is intended to provide a centralized place to perform any checks or
+      # filtering of the command before executing it.
+      #
+      # @param args [Array<String>]
+      # @return [Overcommit::Subprocess::Result] status, stdout, and stderr
       def execute(args)
         if args.include?('|')
           raise Overcommit::Exceptions::InvalidCommandArgs,
                 'Cannot pipe commands with the `execute` helper'
         end
 
-        with_environment 'RUBYOPT' => nil do
-          Subprocess.spawn(args)
-        end
+        Subprocess.spawn(args)
       end
 
       # Calls a block of code with a modified set of environment variables,