Upgrade RBTools (rbt) to latest.

Notably this adds the new `land` subcommand for
potentially smoother patching to mainline master.

Testing Done:
Created this review with `./rbt-create`.

Verified new rbt (land subcommand exists):
```console
$ ./rbt -h | grep land
  land
```

Bugs closed: 961

Reviewed at https://rbcommons.com/s/twitter/r/1629/

.reviewboardrc

@@ -1,2 +1,4 @@
 REVIEWBOARD_URL = 'https://rbcommons.com/s/twitter/'
 REPOSITORY = 'pants'
+LAND_DEST_BRANCH = 'master'
+LAND_SQUASH = True

rbt

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
 source $(dirname $0)/build-support/python/libvirtualenv.sh
-setup_virtualenv 'rbt' 'RBTools==0.6' \
+setup_virtualenv 'rbt' 'RBTools==0.7' \
   '--allow-external RBTools --allow-unverified RBTools'
 exec rbt "$@"