Fix minor version of abomonation crate

`abomonation` 0.7.3 requires a newer Rust compiler.  Change Cargo.toml
to require `0.7.2` instead of `0.7`.

Author: ryzhyk

rust/template/Cargo.toml

@@ -19,7 +19,7 @@ path = "./ovsdb"
 [dependencies]
 differential-dataflow = "0.9"
 timely = "0.9"
-abomonation= "0.7"
+abomonation= "0.7.2"
 serde = "1.0"
 serde_derive = "1.0"
 fnv="1.0.2"

rust/template/differential_datalog/Cargo.toml

@@ -13,7 +13,7 @@ itertools="^0.6"
 
 [dependencies]
 differential-dataflow = "0.9"
-abomonation = "0.7"
+abomonation = "0.7.2"
 timely = "0.9"
 fnv="1.0.2"
 num = "0.2"