Change rebar_ct:check_log/3 to use double quotes

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
failed 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.
Merge pull request #143 from loxybjorn/grep_using_double_quotation_marks

Change rebar_ct:check_log/3 to use double quotes