Don't run linux benchmarks on tags in CI

Author: tsolomko

.travis.yml

@@ -25,7 +25,9 @@ jobs:
       script:
       # On macOS swift test (i.e. XCTest) outputs to stderr so we need to redirect pipes.
       - swift test -c release -Xswiftc -enable-testing --filter BitByteDataBenchmarks 2>&1 | ./ppbenchmarks.py 
-    - language: generic
+    - stage: benchmark
+      if: NOT (tag IS present)
+      language: generic
       os: linux
       dist: trusty
       install: