Run pod linter for one Travis configuration (#33)

* run pod linter for one of the Travis configurations
* use more recent CocoaPods on Travis for pod linting

Author: DivineDominion

.travis.yml

@@ -1,8 +1,5 @@
 language: objective-c
 
-install:
-  - gem install xcpretty
-
 env:
   global:
     - LC_CTYPE=en_US.UTF-8
@@ -16,7 +13,7 @@ matrix:
     - osx_image: xcode10.1
       env: SCHEME="macOS"  SDK="macosx10.14"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="4.2" ACTION="test"
     - osx_image: xcode10.2
-      env: SCHEME="macOS"  SDK="macosx10.14"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="5.0" ACTION="test"
+      env: SCHEME="macOS"  SDK="macosx10.14"          DESTINATION="arch=x86_64"                    SWIFT_VERSION="5.0" ACTION="test" POD_LINT="YES"
     - osx_image: xcode10.2
       env: SCHEME="iOS"    SDK="iphonesimulator"      DESTINATION="OS=12.0,name=iPhone 8"          SWIFT_VERSION="5.0" ACTION="test"
     - osx_image: xcode10.2
@@ -25,8 +22,15 @@ matrix:
       env: SCHEME="watchOS"  SDK="watchsimulator"     DESTINATION="OS=4.0,name=Apple Watch - 42mm" SWIFT_VERSION="5.0" ACTION="build"
 
 before_install:
+  - if [ $POD_LINT == "YES" ]; then
+      gem install cocoapods -v '1.7.5';
+      pod repo update;
+    fi
   - carthage bootstrap --platform "$SCHEME" --verbose
 
+install:
+  - gem install xcpretty
+
 script:
   - set -o pipefail
   - xcodebuild -version
@@ -42,6 +46,9 @@ script:
     GCC_INSTRUMENT_PROGRAM_FLOW_ARCS=YES
     GCC_GENERATE_TEST_COVERAGE_FILES=YES
     SWIFT_VERSION=$SWIFT_VERSION
+  - if [ $POD_LINT == "YES" ]; then
+      pod lib lint --verbose;
+    fi
 
 after_success:
   - bash <(curl -s https://codecov.io/bash) -J ReSwift-Thunk