Replace single quotation marks with double quotation marks in rebar_ct:check_log/3

The call to the grep program in rebar_ct:check_log/3 used single quotation marks
around the strings grep should search for. This works well in most cases but
fails on Windows 7 using GNU grep 2.5.4 as installed by the Chocolatey package
GnuWin 0.6.3.1 with the follow message:

ERROR: cmd /q /c grep -e 'TEST COMPLETE' -e '{error,make_failed}' ct/raw.log
faile with error: 2 and output:
grep: COMPLETE': No such file or directory

This commit changes the single quotation marks to double quotation marks. I've
tested this using GNU grep 2.5.3 on a Debian Linux machine and it works well.