macOS: cover minor versions of xcode

uses subshell for getting xcode version

.travis.yml

@@ -4,6 +4,7 @@ matrix:
   include:
     - compiler: clang
       os: osx
+      osx_image: xcode9.4
       env: DO_PUSH_ARTIFACT=yes
     - compiler: gcc
       os: linux

MacOSX/build-package.in

@@ -103,7 +103,7 @@ osacompile -o "$BUILDPATH/target/Applications/OpenSC Notify.app" "MacOSX/OpenSC_
 
 
 # Build OpenSC.tokend when XCode version < 10
-if (( xcodebuild -version | sed -En 's/Xcode[[:space:]]+([0-9]+)\.[0-9]*/\1/p' < 10 )); then
+if (( $(xcodebuild -version | sed -En 's/Xcode[[:space:]]+([0-9]+)(\.[0-9]*)*/\1/p') < 10 )); then
 	# Check out OpenSC.tokend, if not already fetched.
 	if ! test -e OpenSC.tokend; then
 		git clone http://github.com/OpenSC/OpenSC.tokend.git