remove hardcoding of docker-engine package version

This is needed to allow the travis-ci builds to succeed.
It was failing with errors like this:

```
$ travis_retry sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine=${TRAVIS_DOCKER_VERSION}
Reading package lists... Done
Building dependency tree
Reading state information... Done
Recommended packages:
  yubico-piv-tool
The following packages will be DOWNGRADED:
  docker-engine
0 upgraded, 0 newly installed, 1 downgraded, 0 to remove and 15 not upgraded.
Need to get 8,725 kB of archives.
After this operation, 58.0 MB disk space will be freed.
E: There are problems and -y was used without --force-yes

The command "sudo apt-get -o Dpkg::Options::=--force-confnew install -y docker-engine=1.10.3-0~trusty" failed. Retrying, 2 of 3.

```

.travis.yml

@@ -8,11 +8,10 @@ services:
 - docker
 env:
   global:
-  - TRAVIS_DOCKER_VERSION=1.10.3-0~trusty
   - DOCKER_REPO=$TRAVIS_REPO_SLUG
 before_install:
 - apt-cache madison docker-engine
-- travis_retry sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine=${TRAVIS_DOCKER_VERSION}
+- travis_retry sudo apt-get -o Dpkg::Options::="--force-confnew" install -y docker-engine
 install: mvn install -DskipTests=true -Dmaven.javadoc.skip=true
 script:
 - jdk_switcher use oraclejdk8