[librsvg] Require Rust 1.31, per gtk-rs's new release

[Federico Mena Quintero <federico src gnome org>]

commit 4db23d5f1f14a72e2de006b923320faed730e482
Author: Federico Mena Quintero <federico gnome org>
Date:   Fri Feb 22 09:30:01 2019 -0600

    Require Rust 1.31, per gtk-rs's new release

 COMPILING.md | 2 +-
 configure.ac | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/COMPILING.md b/COMPILING.md
index 0cf220db..2f06050b 100644
--- a/COMPILING.md
+++ b/COMPILING.md
@@ -32,7 +32,7 @@ minimum version is listed here; you may use a newer version instead.
 **Compilers:**
 
 * a C compiler and `make` tool; we recommend GNU `make`.
-* rust 1.30 or later
+* rust 1.31 or later
 * cargo
 
 **Mandatory dependencies:**
diff --git a/configure.ac b/configure.ac
index 6818ebc9..ed92d532 100644
--- a/configure.ac
+++ b/configure.ac
@@ -113,7 +113,7 @@ AS_IF(test x$RUSTC = xno,
 dnl Minimum version of rustc that we support
 dnl If you change this, please update COMPILING.md
 MINIMUM_RUST_MAJOR=1
-MINIMUM_RUST_MINOR=30
+MINIMUM_RUST_MINOR=31
 
 rust_version=`$RUSTC --version`
 version=`echo "$rust_version" | sed -e 's/^rustc //g'`