simplify Travis config to use swift test only

swift test builds JWT and JWA, so compilation errors for those will otherwise just result in two jobs that fail for the same reason; one can still check the Travis output to see where exactly the failure is occurring

Author: jonblatho

.travis.yml

@@ -8,9 +8,9 @@ dist: trusty
 
 env:
   global:
+    - SWIFT_VERSION=4.0
     - SWIFT_VERSION=4.1.2
   matrix:
-    - SWIFTPM_BUILD=true
     - SWIFTPM_TEST=true
     - XCODE_TEST_SDK=macosx
     - XCODE_BUILD_SDK=iphonesimulator
@@ -19,12 +19,6 @@ env:
 
 matrix:
   exclude:
-#     # No need to build and test on macOS
-#     - os: osx
-#       env: SWIFTPM_BUILD=true
-#     # LinuxMain.swift is out of sync
-#     - os: linux
-#       env: SWIFTPM_TEST=true
     - os: linux
       env: XCODE_TEST_SDK=macosx
     - os: linux
@@ -38,7 +32,6 @@ install:
   - eval "$(curl -sL https://swiftenv.fuller.li/install.sh)"
 
 script:
-- if [ -n "$SWIFTPM_BUILD" ]; then swift build; fi
 - if [ -n "$SWIFTPM_TEST" ]; then swift test; fi
 - if [ -n "$XCODE_BUILD_SDK" ] || [ -n "$XCODE_TEST_SDK" ]; then swift package generate-xcodeproj; fi
 - if [ -n "$XCODE_BUILD_SDK" ]; then xcodebuild -project JWT.xcodeproj -scheme JWT-Package build -sdk $XCODE_BUILD_SDK; fi