Adding Bamboo PR support

[atl-mk]

Hey,

Would be keen to see Bamboo support, the variables are listed here: https://confluence.atlassian.com/bamboo/bamboo-variables-289277087.html

[SpanishPear]

Bamboo should be supported, at least it's defined in ci-info https://github.com/watson/ci-info/blob/master/vendors.json#L35

[atl-mk]

Sorry, yes it is, but not PR support which could be done with bamboo_repository_pr_sourceBranch which admittedly will only be present with a PR trigger

[sibiraj-s]

Feel free to open a PR in ci-info repo, should be enough to simply add pr: bamboo_repository_pr_sourceBranch key to json and update readme. Or else I'll try to get this fixed this weekend.