Activate race checks in appveyor

Signed-off-by: Vadim Markovtsev <vadim@sourced.tech>

.appveyor.yml

@@ -23,7 +23,7 @@ build_script:
 
 test_script:
   - go get -v -t -d gopkg.in/src-d/hercules.v9/...
-  - go test -v -tags disable_babelfish gopkg.in/src-d/hercules.v9/...
+  - go test -v -race -tags disable_babelfish gopkg.in/src-d/hercules.v9/...
 
 artifacts:
   - name: hercules.win64.zip