Fix bashism in josm launcher

[sebastic]

The recent Java 9 change (SVN r12713) to the josm launchers introduced a ​bashism, the test only works when bash is configured as /bin/sh:

$ checkbashisms linux/*/usr/bin/josm*
possible bashism in linux/latest/usr/bin/josm-latest line 49 (alternative test command ([[ foo ]] should be [ foo ])):
    if [[ $JAVACMD == *"java-9"* ]]; then
possible bashism in linux/latest/usr/bin/josm-latest line 49 (should be 'b = a'):
    if [[ $JAVACMD == *"java-9"* ]]; then
possible bashism in linux/tested/usr/bin/josm line 49 (alternative test command ([[ foo ]] should be [ foo ])):
    if [[ $JAVACMD == *"java-9"* ]]; then
possible bashism in linux/tested/usr/bin/josm line 49 (should be 'b = a'):
    if [[ $JAVACMD == *"java-9"* ]]; then

The attached patch changes the test to use expr(1) which should be portable to all shells.