tidyr 0.7.1

This is a hotfix release to account for some tidyselect changes in the
unit tests.

Note that the upcoming version of tidyselect backtracks on some of the
changes announced for 0.7.0. The special evaluation semantics for
selection have been changed back to the old behaviour because the new
rules were causing too much trouble and confusion. From now on data
expressions (symbols and calls to : and c()) can refer to both
registered variables and to objects from the context.

However the semantics for context expressions (any calls other than to
: and c()) remain the same. Those expressions are evaluated in the
context only and cannot refer to registered variables. If you're
writing functions and refer to contextual objects, it is still a good
idea to avoid data expressions by following the advice of the 0.7.0
release notes.