update dockerfile to use ruby 2.4.0
update dockerfile to use ruby 2.4.0

[finish #141882339]

Signed-off-by: Tony Wong <3261da024e85c36767bb9324ced93ba4333a2c77@pivotal.io>

bspeck/LicenseFinder,bspeck/LicenseFinder,pivotal/LicenseFinder,pivotal/LicenseFinder,bdshroyer/LicenseFinder,bdshroyer/LicenseFinder,bspeck/LicenseFinder,bdshroyer/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,tinfoil/LicenseFinder,bdshroyer/LicenseFinder,bdshroyer/LicenseFinder,bspeck/LicenseFinder,tinfoil/LicenseFinder,bspeck/LicenseFinder,pivotal/LicenseFinder,pivotal/LicenseFinder,sschuberth/LicenseFinder,pivotal/LicenseFinder,sschuberth/LicenseFinder,tinfoil/LicenseFinder,pivotal/LicenseFinder,tinfoil/LicenseFinder,sschuberth/LicenseFinder,tinfoil/LicenseFinder,sschuberth/LicenseFinder