Gemfile in futex-0.8.5 vs Gemfile in futex-0.8.6

@@ -19,6 +19,7 @@
 # LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 # OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 # SOFTWARE.
 
 source 'https://rubygems.org'
+ruby '2.6.0'
 gemspec