Update composer.json

bootstrap was upgraded to 3.1.0

according to this: https://getcomposer.org/doc/01-basic-usage.md#next-significant-release-tilde-operator-

this change should allow version >=3.0,<4.0

refs https://github.com/components/bootstrap/pull/24
Merge pull request #1 from craigh/patch-1

Update composer.json