Update Travis config to use 10.3.1 simulator.

.travis.yml

@@ -17,7 +17,7 @@ env:
     - DESTINATION="OS=3.2,name=Apple Watch - 42mm" SCHEME="$WATCHOS_FRAMEWORK_SCHEME"  RUN_TESTS="NO"  BUILD_EXAMPLE="NO"  POD_LINT="NO"
     - DESTINATION="OS=2.0,name=Apple Watch - 42mm" SCHEME="$WATCHOS_FRAMEWORK_SCHEME"  RUN_TESTS="NO"  BUILD_EXAMPLE="NO"  POD_LINT="NO"
 
-    - DESTINATION="OS=10.3,name=iPhone 7 Plus"     SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES" BUILD_EXAMPLE="YES" POD_LINT="YES"
+    - DESTINATION="OS=10.3.1,name=iPhone 7 Plus"     SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES" BUILD_EXAMPLE="YES" POD_LINT="YES"
     - DESTINATION="OS=9.0,name=iPhone 6"           SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES" BUILD_EXAMPLE="YES" POD_LINT="NO"
     - DESTINATION="OS=8.1,name=iPhone 4S"          SCHEME="$IOS_FRAMEWORK_SCHEME"      RUN_TESTS="YES" BUILD_EXAMPLE="YES" POD_LINT="NO"