flatbuffers: add 1.10.0 (#10137)

Also unpin the version from the arrow package.

var/spack/repos/builtin/packages/arrow/package.py

@@ -21,7 +21,7 @@ class Arrow(CMakePackage):
 
     depends_on('boost@1.60:')
     depends_on('cmake@3.2.0:', type='build')
-    depends_on('flatbuffers@1.8.0 build_type=Release')  # only Release contains flatc
+    depends_on('flatbuffers build_type=Release')  # only Release contains flatc
     depends_on('python', when='+python')
     depends_on('py-numpy', when='+python')
     depends_on('rapidjson')

var/spack/repos/builtin/packages/flatbuffers/package.py

@@ -13,5 +13,6 @@ class Flatbuffers(CMakePackage):
     homepage = "http://google.github.io/flatbuffers/"
     url      = "https://github.com/google/flatbuffers/archive/v1.9.0.tar.gz"
 
+    version('1.10.0', '3714e3db8c51e43028e10ad7adffb9a36fc4aa5b1a363c2d0c4303dd1be59a7c')
     version('1.9.0', '8be7513bf960034f6873326d09521a4b')
     version('1.8.0', '276cab8303c4189cbe3b8a70e0515d65')