Use openjdk8

To workaround `Failure/Error: Unable to find java.sql.DriverManager.getConnection(java/sql/DriverManager.java to read failed line`

https://travis-ci.org/rsim/ruby-plsql/jobs/558503445

```ruby
1) Connection create and destroy should create connection
     Failure/Error: Unable to find java.sql.DriverManager.getConnection(java/sql/DriverManager.java to read failed line

     Java::JavaSql::SQLException:
       No suitable driver found for jdbc:oracle:thin:@localhost:1521/XE
     # java.sql.DriverManager.getConnection(java/sql/DriverManager.java:702)
     # java.sql.DriverManager.getConnection(java/sql/DriverManager.java:228)
     # jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
     # jdk.internal.reflect.NativeMethodAccessorImpl.invoke(jdk/internal/reflect/NativeMethodAccessorImpl.java:62)
     # jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(jdk/internal/reflect/DelegatingMethodAccessorImpl.java:43)
     # java.lang.reflect.Method.invoke(java/lang/reflect/Method.java:566)
     # RUBY.get_connection(/home/travis/build/rsim/ruby-plsql/spec/spec_helper.rb:89)
     # RUBY.try_to_connect(/home/travis/build/rsim/ruby-plsql/spec/spec_helper.rb:100)
     # RUBY.get_connection(/home/travis/build/rsim/ruby-plsql/spec/spec_helper.rb:88)
     # RUBY.<main>(/home/travis/build/rsim/ruby-plsql/spec/plsql/connection_spec.rb:8)
     # java.lang.invoke.MethodHandle.invokeWithArguments(java/lang/invoke/MethodHandle.java:710
```

Actually I do not know the reason why yet.
I just found these Java version differences between Trusty and Xenial,
then installed `openjdk8` on Xenial.

* Trusty
```
$ java -Xmx32m -version
java version "1.8.0_151"
```

* Xenial
```
$ java -Xmx32m -version
openjdk version "11.0.2" 2019-01-15
```

* Xenial with this pull request
```
$ java -Xmx32m -version
openjdk version "1.8.0_191"
```

Author: yahonda

.travis.yml

@@ -1,6 +1,8 @@
 dist: xenial
 sudo: required
 
+jdk:
+  - openjdk8
 env:
   global:
     - ORACLE_COOKIE=sqldev