Update syn to 0.15

Author: Guillaume Fraux

soa-derive-internal/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "soa_derive_internal"
-version = "0.6.1"
+version = "0.6.2"
 authors = ["Guillaume Fraux <guillaume.fraux@chimie-paristech.fr>"]
 license = "MIT/Apache-2.0"
 readme = "../README.md"
@@ -15,6 +15,6 @@ Internal crate implementation for soa-derive
 proc-macro = true
 
 [dependencies]
-syn = "0.14"
+syn = "0.15"
 quote = "0.6"
 proc-macro2 = "0.4"