Missing PHP version in constructed Travis file

[raphaelstolt]

I built a project on PHP 5.6.16 and in the constructed Travis file an entry for 5.6 is missing in the versions list.

Test case to reproduce:

    /**
     * @test
     * @ticket 91 (https://github.com/jonathantorres/construct/issues/91)
     */
    public function it_should_return_all_versions_to_test_on_a_php5616_project()
    {
        $versionsToTest = $this->travis->phpVersionsToTest('5.6.16');

        $versionsExpected = [
            'hhvm',
            'nightly',
            '5.6',
            '7.0',
        ];

        $this->assertEquals($versionsToTest, $versionsExpected);
    }