folly/test/RangeTest.cpp: avoid -Wsign-compare error (trivial)

folly/test/RangeTest.cpp: avoid -Wsign-compare error (trivial)

Summary:
* folly/test/RangeTest.cpp (NeedleFinderTest): Declare for-loop
index to be of type size_t.  OTherwise, gcc-4.9 fails:
folly/test/RangeTest.cpp:910:23: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]

Test Plan:
Run this and note there are fewer errors than before:
fbconfig --platform-all=gcc-4.9-glibc-2.20 -r folly && fbmake dbgo

Reviewed By: philipp@fb.com

Subscribers: folly-diffs@

FB internal diff: D1768279

Tasks: 5941250

Signature: t1:1768279:1420593237:246040eebd40e313bdb19799bb11d6639f1481de