Switch Travis CI build env to Ubuntu Xenial (16.04) (#337)
Switch Travis CI build env to Ubuntu Xenial (16.04) (#337)

* Switch build env to Ubuntu Xenial (16.04)

* Remove addons

Xenial build image already includes g++ and autoconf packages so we
don't need to explicitly install them.

aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs,aerospike/aerospike-client-nodejs