Update linux-tests; fail if warnings on travis

* Update linux-tests; fail if warnings on travis

* Fix warnings on Linux

* Typo

* Can’t test these on Linux

.travis.yml

@@ -16,18 +16,21 @@ xcode_scheme: Path.swift-Package
 jobs:
   include:
     - name: macOS / Swift 4.0.3
+      before_script: swift build -Xswiftc -warnings-as-errors
       script: swift test --parallel -Xswiftc -swift-version -Xswiftc 4
 
-    - name: macOS / Swift 4.2.1
+    - &std
+      name: macOS / Swift 4.2.1
+      before_script: swift build -Xswiftc -warnings-as-errors
       script: swift test --parallel
 
-    - name: macOS / Swift 5.0
+    - <<: *std
+      name: macOS / Swift 5.0
       osx_image: xcode10.2
-      script: swift test --parallel
 
-    - name: macOS / Swift 5.1
+    - <<: *std
+      name: macOS / Swift 5.1
       osx_image: xcode11
-      script: swift test --parallel
 
     - &xcodebuild
       before_install: swift package generate-xcodeproj --enable-code-coverage
@@ -57,6 +60,7 @@ jobs:
       language: generic
       sudo: false
       install: eval "$(curl -sL https://swiftenv.fuller.li/install.sh)"
+      before_script: swift build -Xswiftc -warnings-as-errors
       script: swift test --parallel
 
     - <<: *linux