fix: bootstrap 3.1+ compatibility #147

I was digging around a bit and found a fix (hopefully ;))
I've checked it with most browsers including IE 9+ and all major versions of Bootstrap 3.
Everything works as intended.

There were two fixes in Bootstrap which affected minicolors:
z-index: https://github.com/twbs/bootstrap/commit/eb20d6f13244052f7794825531c9ee7e76430559
float: https://github.com/twbs/bootstrap/commit/432b9f9cde8e2b067fd54ed148c5f8df510e30c5

The swatch was not visible in 3.3.1 anymore, as Bootstrap changed the z-index of the input field to 2.
I changed the z-index in the bootstrap-theme to 2 as well.

The positioning issue was related to the input field's float:left; introduced in Bootstrap 3.1.0.
The input's floating is now overwritten to none.