Remove bash specific calls

Make sure that every call in a shell script is supported by /bin/sh and not a bashism, like $()

Closes #4314

bin/elasticsearch

@@ -89,7 +89,7 @@ fi
 if [ -x "$JAVA_HOME/bin/java" ]; then
     JAVA="$JAVA_HOME/bin/java"
 else
-    JAVA=$(which java)
+    JAVA=`which java`
 fi
 
 if [ ! -x "$JAVA" ]; then

src/rpm/init.d/elasticsearch

@@ -53,7 +53,7 @@ checkJava() {
     if [ -x "$JAVA_HOME/bin/java" ]; then
         JAVA="$JAVA_HOME/bin/java"
     else
-        JAVA=$(which java)
+        JAVA=`which java`
     fi
 
     if [ ! -x "$JAVA" ]; then