travis.yml: Use BREW_INSTALL_PACKAGES to install libressl

Author: Pushkar N Kulkarni

.travis.yml

@@ -50,10 +50,9 @@ matrix:
     - os: osx
       osx_image: xcode10.1
       sudo: required
-      env: SWIFT_SNAPSHOT=4.2.1 KITURA_NIO=1
+      env: SWIFT_SNAPSHOT=4.2.1 KITURA_NIO=1 BREW_INSTALL_PACKAGES="libressl"
 
 before_install:
-  - if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then brew update && brew install libressl ; fi
   - git clone https://github.com/IBM-Swift/Package-Builder.git
 
 script: