Remove usage of com.oracle.net package

[pbludov]

This package is unavailable in the OpenJDK9 (but is available in the OracleJDK9).
The only usage of this package is the file src/test/resources/com/puppycrawl/tools/checkstyle/checks/whitespace/emptylineseparator/InputEmptyLineSeparator.java.

For that test the package name is irrelevant and can be changed to any other.

[romani]

it might be also good to put in Travis build for openjdk, to prevent problems like this in future and never let anybody create new problem like this.