Don't warn when there are no source files.

This is an expected result, e.g., when using wildcards, or something like
https://github.com/tschaub/grunt-newer that only removes unchanged files from
the source list.
Merge branch 'fix-warning' of https://github.com/russelldavis/grunt-text-replace into russelldavis-fix-warning
Accepted pull request, and removed trace of reporting warning when no source files found
0.3.10
Update to docs