adjust java 7 build (to see if it fixes travis build)

[pjfanning]

The travis build is failing for scala 2.10 and 2.11. This PR changes to use the travis trusty distribution.

[pjfanning]

Merged to get travis build passing - will make any changes that anyone suggests, if they spot anything wrong in this PR.

[cowtowncoder]

Excellent! And removing Java 7 code is fine, I think: builds need JDK 8 regardless but I don't think Kotlin or Scala even work well on JDK 7 these days.

Although README should mention this if not already pointed out, as baseline for Jackson 2.x otherwise is JDK 7 generally (and JDK 6 for annotations, streaming; JDK 8 for Java 8 module)

[pjfanning]

@cowtowncoder scala 2.12 and 2.13 need java 8 as a minimumum but scala 2.10 and 2.11 should work with java 6 (if we need it too)

[cowtowncoder]

@pjfanning Ok. I am fine either way, as long as we document this.

On usage, Sonatype OSS Nexus gives d/l rates of:

• 2.13: 2k
• 2.12: 280k
• 2.11: 448k
• 2.10: 68k
• 2.9 : < 1k

so I guess theoretically 2.11 on pre-Java 8 might be a thing. But we can only publish on JDK 8+, at least with Maven (not sure what SBT does).