Fix Plasma starting failure when specify the memory in float value. (#2337)

python/ray/plasma/plasma.py

@@ -60,6 +60,9 @@ def start_plasma_store(plasma_store_memory=DEFAULT_PLASMA_STORE_MEMORY,
         raise Exception("If huge_pages is True, then the "
                         "plasma_directory argument must be provided.")
 
+    if not isinstance(plasma_store_memory, int):
+        raise Exception("plasma_store_memory should be an integer.")
+
     plasma_store_executable = os.path.join(
         os.path.abspath(os.path.dirname(__file__)),
         "../core/src/plasma/plasma_store")