[Fix #113] Add a Ruby checker using RuboCop
Add configuration file variable for RuboCop [#113]
Fix syntax in README
Fix comments

Remove outdated comments, and move comments to right locations.
Un-declare false command
Add command to copy error messages at point
Fix highlighting and navigation in narrowed buffers
Fix navigation to first error

Regression from 5136836
Fix the name of the Rust syntax checker

The syntax checker uses the standard Rust compiler, so there is no need
to a tool name postfix.
Use the right buffer to determine the error region

The error region must be determined in the buffer associated with the
error.
Remove out-dated TODO item
Fix type error

Regression from 4a9e4c3
Fix Rust checker references

Regression from 42aa87c
Fix verification of `eval` cell

Thanks, @ptrv, for bringing this to my attention.
Add test cases for declaration verification
Use error patterns in php-phpcs

Parsing XML is nicer in theory, but in practice PHP has notoriously
unstable output habits which tend to break XML parsing.

See #78 and #118.
[Fix #115] Remove Pyflakes

Flake8 is superior in all aspects
Improve speed of Python provisioning

Use a simpler index to avoid expensive link crawling on the standard index.
Improve wording in changelog

[ci skip]
Build docs on Travis

Ensures that our docs build cleanly