Update for libsyntax API change.
Update for libsyntax API change.

In rust-lang/rust@e7d3ae606ed496144554dae499b69207da3b09c5
`parse_expr()` was renamed to `parse_expr_panic()`. This commit reflects
that change.

sfackler/rust-postgres-macros,zsiciarz/rust-postgres-macros,sfackler/rust-postgres-macros,zsiciarz/rust-postgres-macros