extern: always use java version 1.7

This is required because the fdroidclient app also explicitly sets java
version to 1.7 and the modules need to be compiled for the same version.

extern/nanohttpd/core/build.gradle

@@ -2,6 +2,9 @@ apply plugin: 'java'
 
 version = '2.1.0'
 
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
+
 jar {
     baseName = 'nanohttpd'
 }

extern/zipsigner/build.gradle

@@ -4,6 +4,9 @@ repositories {
     jcenter()
 }
 
+sourceCompatibility = 1.7
+targetCompatibility = 1.7
+
 dependencies {
     compile 'com.madgag.spongycastle:pkix:1.53.0.0'
     compile 'com.madgag.spongycastle:prov:1.53.0.0'