Allow defining SELENIUM_VERSION on env.

Updated to Selenium 2.31.0 and added support for defining the version as an environment variable
Passing tests by removing --ignore-leaks which is no longer valid on mocha 1.8.2

Now in order to override the version you need to supply a valid sha.

This also fixes the invalid sha for 2.31.0 and removes

SELENIUM_VERSION=2.28.0:360e3d1d2096c5bcfb83faf44997f700bdc6d289 npm test
Removing duplicated version from filename
Merge pull request #4 from bermi/master

Alternative to hardcoded version via an environment variable.