Red test to expose ' ' being prepended to message subject
Refactor `prepend_subject_array` to eliminate prepended whitespace bug

A bug was introduced where a single white space was being added to a
subject line when neither of the prepend configurations were enabled.
This commit refactors the `prepend_custom_subject` method to add a
space only when there are strings to prepend. It also introduces a new
convenience method `custom_subject` which does the array join
previously done in `prepend_custom_subject`. It also reduces the
complexity of the method itself, which was introduced before to better
handle messages with a `nil` subject.

This also makes the test expectation in the previous commit pass.
Remove Ruby-1.9.2 from TravisCI config

- removes 1.9.2 as it’s not supported by some of the dependent gems

- adds 2.1.0 to begin testing latest release of Ruby
Add Ruby 2.1.5 to Travis config
Merge pull request #23 from rochers/master

Eliminate prepended whitespace when neither prepend configurations are set to true