Update README
Update CHANGELOG for Release 7.4.0
update rvm
correct the url used to install gvm
update all github raw urls
add the ability to install the latest rvm minor
oops, forgot a commit
Install unixodbc for Erlang packages.

This enables Erlang to be compilex with built-in ODBC support.

Fixes travis-ci/travis-ci#2254
Merge pull request #308 from travis-ci/gc-changelog-releases

Update CHANGELOG for Release 7.4.0
Merge pull request #292 from travis-ci/readme

Update README
Merge pull request #310 from travis-ci/mm-erlang-unixodbc

Install unixodbc for Erlang packages
Don't preinstall old versions of Scala and sbt

Note: sbt 0.13.1 is still pre-installed as it is the default version of
the current sbt-extras launcher script.
Add Erlang/ODBC change to changes.
Fix breaking change related to hhvm-switcher.

The PATH to phpenv is now absolute in hhvm-switcher. It should prevent
`phpenv: command not found` errors in scripts run with sudo.

Fixes travis-ci/travis-ci#2235 and travis-ci/travis-ci#2261.