Put therubyracer in the Gemfile

therubyracer is a JS runtime required by execjs, which is in turn required
by uglifier. therubyracer is not the fastest option but it is the most portable.

Doing this makes Kochiku easier to use by newcomers because they do not
need to install Node or another JS runtime on their machines.
Allow disabling kochiku merges on a repo by repo basis.

Controlled by a new column on Repository 'allows_kochiku_merges'.
Rename build auto_merge attribute to merge_on_success
Rename the AutoMergeMailer to MergeMailer
Rename GitAutomerge to GitMergeExecutor