Update lowest supported version of polyfill
diff --git a/README.md b/README.md
index 3632fb9..5f920a7 100644
--- a/README.md
+++ b/README.md
@@ -60,10 +60,10 @@
proc-macro2 by default.
To opt into the additional APIs available in the most recent nightly compiler,
-the `procmacro2_semver_exempt` config flag must be passed to rustc. As usual, we
-will polyfill those nightly-only APIs all the way back to Rust 1.15.0. As these
-are unstable APIs that track the nightly compiler, minor versions of proc-macro2
-may make breaking changes to them at any time.
+the `procmacro2_semver_exempt` config flag must be passed to rustc. We will
+polyfill those nightly-only APIs back to Rust 1.31.0. As these are unstable APIs
+that track the nightly compiler, minor versions of proc-macro2 may make breaking
+changes to them at any time.
```
RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
diff --git a/src/lib.rs b/src/lib.rs
index 6c896e0..b10eef1 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -56,9 +56,9 @@
//!
//! To opt into the additional APIs available in the most recent nightly
//! compiler, the `procmacro2_semver_exempt` config flag must be passed to
-//! rustc. As usual, we will polyfill those nightly-only APIs all the way back
-//! to Rust 1.15.0. As these are unstable APIs that track the nightly compiler,
-//! minor versions of proc-macro2 may make breaking changes to them at any time.
+//! rustc. We will polyfill those nightly-only APIs back to Rust 1.31.0. As
+//! these are unstable APIs that track the nightly compiler, minor versions of
+//! proc-macro2 may make breaking changes to them at any time.
//!
//! ```sh
//! RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build