Update Java SDK dependencies

[angelomelonas]

Most of our dependencies are very out of date. The following version changes will be made:

• junit: 4.12 -> 4.13
• gson: 2.8.1 -> 2.8.6
• okhttp: 3.9.1 -> 3.14.9
• commons-io: 1.3.2 -> 2.7
• okio: 1.13.0 -> 1.17.5
• jaxb-api: 2.3.0 -> 2.3.1
• jaxb-core: 2.3.0 -> 2.3.0.1
• jaxb-impl: 2.3.0 -> 2.3.3

Thanks to @Nayels for pointing out that our dependencies are out of date.