Upgrades to latest gradle

I'd like to use the relatively new Maven BOM support to revamp the
Zipkin dependencies after this change. 4.6 is strictly the minimum,
but figured I'd go for latest.

gradle/wrapper/gradle-wrapper.properties

@@ -1,6 +1,5 @@
-#Wed Sep 06 16:34:21 PDT 2017
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-4.8-bin.zip

gradlew

@@ -33,11 +33,11 @@ DEFAULT_JVM_OPTS=""
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD="maximum"
 
-warn ( ) {
+warn () {
     echo "$*"
 }
 
-die ( ) {
+die () {
     echo
     echo "$*"
     echo
@@ -155,7 +155,7 @@ if $cygwin ; then
 fi
 
 # Escape application args
-save ( ) {
+save () {
     for i do printf %s\\n "$i" | sed "s/'/'\\\\''/g;1s/^/'/;\$s/\$/' \\\\/" ; done
     echo " "
 }