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

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

.travis.yml

@@ -19,6 +19,12 @@ os:
   - linux
   - osx
 
+cache:
+  apt: true
+  directories:
+    - .build/checkouts
+    - .build/repositories
+
 # Use Ubuntu 14.04
 dist: trusty
 
@@ -29,23 +35,25 @@ sudo: false
 addons:
   apt:
     packages:
-    - clang-3.8 
-    - lldb-3.8 
-    - libicu-dev 
-    - libtool 
-    - libcurl4-openssl-dev 
-    - libbsd-dev 
-    - build-essential 
-    - libssl-dev 
-    - uuid-dev
-    - curl
-    - unzip
+      - clang-3.8 
+      - lldb-3.8 
+      - libicu-dev 
+      - libtool 
+      - libcurl4-openssl-dev 
+      - libbsd-dev 
+      - build-essential 
+      - libssl-dev 
+      - uuid-dev
+      - curl
+      - unzip
 
 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