Update travis config to xcode 11.6 (fixes #4)

Author: daprice

.travis.yml

@@ -2,13 +2,13 @@
 # * https://www.objc.io/issues/6-build-tools/travis-ci/
 # * https://github.com/supermarin/xcpretty#usage
 
-osx_image: xcode11.3
+osx_image: xcode11.6
 language: swift
 # cache: cocoapods
 # podfile: Example/Podfile
 # before_install:
 # - gem install cocoapods # Since Travis is not always on latest version
 # - pod install --project-directory=Example
 script:
-- set -o pipefail && xcodebuild test -enableCodeCoverage YES -workspace Example/TactileSlider.xcworkspace -scheme TactileSlider-Example -sdk iphonesimulator13.2 -destination "platform=iOS Simulator,OS=13.3,name=iPhone 11" ONLY_ACTIVE_ARCH=NO | xcpretty
+- set -o pipefail && xcodebuild test -enableCodeCoverage YES -workspace Example/TactileSlider.xcworkspace -scheme TactileSlider-Example -sdk iphonesimulator13.6 -destination "platform=iOS Simulator,OS=13.6,name=iPhone 11" ONLY_ACTIVE_ARCH=NO | xcpretty
 - pod lib lint