(feat): upgrade to source compatibility to java 8. finally dropping 7 (#264)

* upgrade to source compatibility to java 8.  finally dropping 7

* update readme to mention java 8

Author: thomaszurkan-optimizely

README.md

@@ -11,7 +11,7 @@ This repository houses the Java SDK for Optimizely's Full Stack product.
 
 #### Gradle
 
-The SDK is available through Bintray and is created with source and target compatibility of 1.7. The core-api and httpclient Bintray packages are [optimizely-sdk-core-api](https://bintray.com/optimizely/optimizely/optimizely-sdk-core-api)
+The SDK is available through Bintray and is created with source and target compatibility of 1.8. The core-api and httpclient Bintray packages are [optimizely-sdk-core-api](https://bintray.com/optimizely/optimizely/optimizely-sdk-core-api)
 and [optimizely-sdk-httpclient](https://bintray.com/optimizely/optimizely/optimizely-sdk-httpclient) respectively. To install, place the
 following in your `build.gradle` and substitute `VERSION` for the latest SDK version available via Bintray:
 
@@ -130,4 +130,4 @@ License (Apache 2.0): [https://github.com/google/gson/blob/master/LICENSE](https
 
 **Apache HttpClient** [https://hc.apache.org/httpcomponents-client-ga/index.html ](https://hc.apache.org/httpcomponents-client-ga/index.html)  
 Copyright © January 2004
-License (Apache 2.0): [https://github.com/apache/httpcomponents-client/blob/master/LICENSE.txt](https://github.com/apache/httpcomponents-client/blob/master/LICENSE.txt)
+License (Apache 2.0): [https://github.com/apache/httpcomponents-client/blob/master/LICENSE.txt](https://github.com/apache/httpcomponents-client/blob/master/LICENSE.txt)

build.gradle

@@ -50,8 +50,8 @@ subprojects {
     apply plugin: 'me.champeau.gradle.jmh'
     apply plugin: 'nebula.optional-base'
 
-    sourceCompatibility = 1.7
-    targetCompatibility = 1.7
+    sourceCompatibility = 1.8
+    targetCompatibility = 1.8
 
     repositories {
         jcenter()