Add support for CI masked variables

[PhilippeVienne]

Refer to issue #387

[gmessner]

@PhilippeVienne
Thanks for your contribution.

Just one small request, I noticed there is not a bean test for the variable details. Could you add the following file to src/test/resources/org/gitlab4j/api:

project-variable-details.json

{
    "key": "TEST_VARIABLE_1",
    "variable_type": "env_var",
    "value": "TEST_1",
    "protected": false,
    "masked": true
}

And then the following test to TestGitLabApiBeans.java:

    @Test
    public void testProjectVariableDetails() throws Exception {
        Variable variable = unmarshalResource(Variable.class, "project-variable-details.json");
        assertTrue(compareJson(variable, "project-variable-details.json"));
    }

[gmessner]

@PhilippeVienne
No need to name this isMasked, just name it masked and remove the @JsonProperty("masked") line.

The reason isProtected was used is because protected is a reserved keyword in Java.

[gmessner]

@PhilippeVienne
Not certain on this but I believe variable.getMasked() will return null in this case.

[PhilippeVienne]

Yes correct and that's why a second commit remove it

[gmessner]

@PhilippeVienne
Getting ready to do a release, I'm going to go ahead and merge this and then add the test I mention in my first commit. This will be included in the upcoming release.