MINOR: Update to Gradle 2.9 and update generated `gradlew` file

More performance improvements:

"In many cases, Gradle 2.9 is much faster than Gradle 2.8 when performing incremental builds.

Very large builds (many thousands of source files) could see incremental build speeds up to 80% faster than 2.7 and up to 40% faster than 2.8.

Gradle now uses a more efficient mechanism to scan the filesystem, making up-to-date checks significantly faster. This improvement is only available when running Gradle with Java 7 or newer.

Other improvements have been made to speed-up include and exclude pattern evaluation; these improvements apply to all supported Java versions.

Gradle now uses much less memory than previous releases when performing incremental builds. By de-duplicating Strings used as file paths in internal caches, and by reducing the overhead of listing classes under test for Java projects, some builds use 30-70% less memory that Gradle 2.8."

https://docs.gradle.org/current/release-notes

Author: Ismael Juma <ismael@juma.me.uk>

Reviewers: Grant Henke, Guozhang Wang

Closes #549 from ijuma/gradle-2.9

build.gradle

@@ -45,7 +45,7 @@ allprojects {
 }
 
 ext {
-    gradleVersion = "2.8"
+    gradleVersion = "2.9"
     buildVersionFileName = "kafka-version.properties"
 
     userMaxForks = project.hasProperty('maxParallelForks') ? maxParallelForks.toInteger() : null

gradlew

@@ -42,11 +42,6 @@ case "`uname`" in
     ;;
 esac
 
-# For Cygwin, ensure paths are in UNIX format before anything is touched.
-if $cygwin ; then
-    [ -n "$JAVA_HOME" ] && JAVA_HOME=`cygpath --unix "$JAVA_HOME"`
-fi
-
 # Attempt to set APP_HOME
 # Resolve links: $0 may be a link
 PRG="$0"
@@ -61,9 +56,9 @@ while [ -h "$PRG" ] ; do
     fi
 done
 SAVED="`pwd`"
-cd "`dirname \"$PRG\"`/" >&-
+cd "`dirname \"$PRG\"`/" >/dev/null
 APP_HOME="`pwd -P`"
-cd "$SAVED" >&-
+cd "$SAVED" >/dev/null
 
 CLASSPATH=$APP_HOME/gradle/wrapper/gradle-wrapper.jar
 
@@ -114,6 +109,7 @@ fi
 if $cygwin ; then
     APP_HOME=`cygpath --path --mixed "$APP_HOME"`
     CLASSPATH=`cygpath --path --mixed "$CLASSPATH"`
+    JAVACMD=`cygpath --unix "$JAVACMD"`
 
     # We build the pattern for arguments to be converted via cygpath
     ROOTDIRSRAW=`find -L / -maxdepth 1 -mindepth 1 -type d 2>/dev/null`