Add .gitignore for //tools/luci-go/.versions/isolate.cipd_version

Bug: 851596
Change-Id: Id4c18ac2be3c5d05b426f4a0f84a9c46b12352e0
Reviewed-on: https://chromium-review.googlesource.com/1137707Reviewed-by: Marc-Antoine Ruel <maruel@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#575139}

tools/luci-go/.gitignore

@@ -3,3 +3,4 @@
 /linux64/isolate
 /mac64/isolate
 /win64/isolate.exe
+.versions