Ensure that the phantomjs binary has appropriate UNIX mode.

The 755 mode corresponds to a UNIX mode string of -rwxr-xr-x.

Closes #166.
Don't require `tmp` module unless we're going to use it.
Merge pull request #167 from benjamn/issue-166-fix-phantomjs-executability

Ensure that the phantomjs binary has appropriate UNIX mode