Upgrade to RBTools latest.

Testing Done:
Posted this review using `./rbt`.

For full disclosure, actually:
```console
./rbt post \
  -o \
  --target-groups=pants-reviews \
  --branch="$(git branch | grep "*" | cut -d " " -f2)" \
  --description="$(git slog -1 --format=format:%b)" \
  --summary="$(git log -1 --pretty=format:%s)" \
  --bugs=1676 \
  --parent=HEAD~1
```

CI went green here:
  https://travis-ci.org/pantsbuild/pants/builds/66607040

Bugs closed: 1676

Reviewed at https://rbcommons.com/s/twitter/r/2360/

rbt

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
 source $(dirname $0)/build-support/python/libvirtualenv.sh
-setup_virtualenv 'rbt' 'RBTools==0.7.2' \
-  '--allow-external RBTools --allow-unverified RBTools'
+setup_virtualenv 'rbt' 'RBTools==0.7.4' \
+  '--allow-external RBTools'
 exec rbt "$@"