Using Bower verify to ensure bootstrap is compatible with jquery versions

Commit messages like [this](https://github.com/twbs/bootstrap/commit/6387d8d5cccca1af3767258d6b38394c9f89afae) and other like it in different projects inspired me to create [grunt-bower-verify](https://github.com/mokkabonna/grunt-bower-verify) that installs all tagged versions of bower dependencies and runs tasks after each iteration.

This allows bower package creators to be liberal when defining their dependencies, and confident that the automated testing will reveal any incompatibilities due to changes in their package or in their dependencies.

I have setup bootstrap to run this full test for each jquery version in travis only. Also put a lower version of jquery, as it is in fact compatible with jquery 1.7 (not 1.6 as it does not have `on` for events)

See the summary of the testing on this line: https://travis-ci.org/mokkabonna/bootstrap/builds/11639893#L1527

I had to reference the bower installed version of jquery in the test, and deleted the old one. This means that running the test locally will not work unless you first run bower install. This could also be automated if preferred, ensuring bower dependencies are installed on each run of grunt test, using grunt-bower-task, or simply solved with a simple update to the readme that for development bower is required after checkout.

If this PR is accepted, let me know which approach is preferable and I'll implement that as well.