pin flatbuffers to 0.8.4 (#239)

* pin flatbuffer to 0.8.4

* =0.8.2
diff --git a/arrow/Cargo.toml b/arrow/Cargo.toml
index f23217b..e6773d9 100644
--- a/arrow/Cargo.toml
+++ b/arrow/Cargo.toml
@@ -47,7 +47,7 @@
 lazy_static = "1.4"
 packed_simd = { version = "0.3.4", optional = true, package = "packed_simd_2" }
 chrono = "0.4"
-flatbuffers = "^0.8"
+flatbuffers = "=0.8.4"
 hex = "0.4"
 prettytable-rs = { version = "0.8.0", optional = true }
 lexical-core = "^0.7"