Add java 7 compatibility

[OGKevin]

Steps to reproduce:

1. Most android SDK's don't support java 8
2. Try to use the bunq sdk on one of these android sdk's

What should happen:

1. It works smoothly

What happens:

1. A bunch of errors

Logs

Error:com.android.builder.dexing.DexArchiveBuilderException: Failed to process C:\Users\lenvovo\.gradle\caches\modules-2\files-2.1\com.github.bunq\sdk_java\b5c97e4\e70d9e73b467b9e46cd315e2d40182b175385b38\sdk_java-b5c97e4.jar

Error:com.android.builder.dexing.DexArchiveBuilderException: Error while dexing com/bunq/sdk/security/SecurityUtils.class

Error:com.android.dx.cf.code.SimException: invalid opcode ba (invokedynamic requires --min-sdk-version >= 26)

Error:Execution failed for task ':app:transformClassesWithDexBuilderForDebug'.

> com.android.build.api.transform.TransformException: com.android.builder.dexing.DexArchiveBuilderException: com.android.builder.dexing.DexArchiveBuilderException: Failed to process C:\Users\lenvovo\.gradle\caches\modules-2\files-2.1\com.github.bunq\sdk_java\b5c97e4\e70d9e73b467b9e46cd315e2d40182b175385b38\sdk_java-b5c97e4.jar

Extra info:

• Tested on 0.12.3

[tubbynl]

if this is a thing you can set sourceCompatibility = 1.7 in build.gradle for build-time validation

this project does contain some java8 here and there :)

[tubbynl]

if u want i can create a branch from add_java_7_support_bunq/sdk_java#42 and fix that

but it will probably only make sense to also merge / sync some pending pull requests before that and/or i'll merge them for you in one pull request @OGKevin

[OGKevin]

I've already started working on this, feel free to finish it however. You can do this by creating your own branch as child of my branch which is https://github.com/bunq/sdk_java/compare/add_java_7_support_bunq/sdk_java%2342 and then create a PR that merges into my branch. Afterwards this gets merged in develop.