Update .travis.yml

use `stage: build` for the docker builds to get some parallelism.

.travis.yml

@@ -117,7 +117,7 @@ jobs:
         - pip3 install -e .
 
     - if: tag IS present
-      stage: docker
+      stage: build
       name: "Wallet Server Docker Image - Tagged Release"
       script:
           - set -e
@@ -125,7 +125,7 @@ jobs:
           - travis_retry docker build -t lbry/wallet-server:$TRAVIS_TAG -f scripts/Dockerfile.wallet_server .
           - docker push lbry/wallet-server:$TRAVIS_TAG
     - if: tag IS blank AND branch = master
-      stage: docker
+      stage: build
       name: "Wallet Server Docker Image - Master"
       script:
           - set -e