Try making Travis cache the SwiftPM dependencies, for faster CI builds.

(Also separate the dependency resolution and main build steps into separate steps, so we can time each one individually.)

Author: MrMage

.travis.yml

@@ -19,6 +19,11 @@ os:
   - linux
   - osx
 
+cache:
+  directories:
+    - .build/checkouts
+    - .build/repositories
+
 # Use Ubuntu 14.04
 dist: trusty
 
@@ -46,6 +51,8 @@ install: ./.travis-install.sh
 script: 
   - export PATH=$HOME/local/bin:$PATH
   - export LD_LIBRARY_PATH=$HOME/local/lib
+  - swift package -v resolve
+  - make all
   - make test
   - make test-plugin
   - make test-echo