Add test enviroment to bundler setup on tests

This made possible to run individual tests with something like:

ruby  -Itest/ test/test_spinning_cursor.rb -v -n "/allow you to change the banner/"

Necessary to faster repeating individual tests.
Fix Issue #17

It was just a "timing" issue.
Delaying sometimes was falling before and sometimes after banner changing.
Fix Issue #18

At the SpinningCursor#stop method the 'release_console'
was being called while the "@spinner" was still spinning.
So that the @spinner tries to call $stdout.string after console released
and there's no 'string' method for the STDOUT (as $stdout points to it when
"released")

Inverting the order and killing the spinning before releasing console
fix the issue.
Merge pull request #20 from abinoam/issue17

Fix issue #17 (non-deterministic test due to race conditions)
Merge pull request #21 from abinoam/issue18

Fix Issue #18 (non-deterministic test)
Fix overflowing banner repeating bug (issue #19)

If the banner spans larger than the width of the terminal, the banner
repeats on multiple lines. This commit changes
SpinningCursor::ConsoleHelpers.reset_line to clear overflowing lines and
move the cursor back to the starting line before printing again.

Also adds minitest gem to Gemfile because I needed it there to run tests
on my work machine.

This possibly breaks things for Windows (hah) due to more ANSI escape
sequence fun :).