docs(README): changed coveralls badge to target the master branch

Travis commit : build 244

Merge pull request angular-ui#200 from thgreasi/v0.12.8dev

chore: merge version v0.12.8 into master

Travis commit : build 238

Merge pull request angular-ui#196 from thgreasi/v0.12.7dev

chore: increase version number to v0.12.7

Travis commit : build 218

Merge pull request angular-ui#168 from thgreasi/master

fix(sortable): support `helper` option when set to a function

Travis commit : build 214

Merge pull request angular-ui#165 from thgreasi/wrong-script-order

feat(sortable): log error if jquery was not available to angular on load

Travis commit : build 209