Trim white space when flushing assertions

[gazayas]

Fixes #51.

pry(#<BasicsTest>)> "#{last["action"]} #{last["target"]}#{last["options"]}" + "\n"
=> "assert page.has_content? 'Foo was successfully created.' \n"

Since we're tagging on a new line at the end with a +, just calling strip here should suffice.