Make use and switch able to take a major version as argument

If a major version is given as argument, use/switch will use the latest
available minor version of this major version

e.g.: You have php 5.5.6 and 5.5.9, `phpbrew use 5.5` will be
equivalent to `phpbrew use 5.5.9`
Merge pull request #205 from chadrien/develop

Make use and switch able to take a major version as argument