Revert bundled curl back to 7.68.0

Looks like 7.69.0 had a bug affecting Cargo (rust-lang/cargo#7974), so
let's wait for the next release.

Author: alexcrichton

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "curl"
-version = "0.4.26"
+version = "0.4.27"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT"
 repository = "https://github.com/alexcrichton/curl-rust"
@@ -16,7 +16,7 @@ appveyor = { repository = "alexcrichton/curl-rust" }
 
 [dependencies]
 libc = "0.2.42"
-curl-sys = { path = "curl-sys", version = "0.4.27", default-features = false }
+curl-sys = { path = "curl-sys", version = "0.4.28", default-features = false }
 socket2 = "0.3.7"
 
 # Unix platforms use OpenSSL for now to provide SSL functionality

curl-sys/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "curl-sys"
-version = "0.4.28+curl-7.69.0"
+version = "0.4.29+curl-7.68.0"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 links = "curl"
 build = "build.rs"

curl-sys/build.rs

@@ -175,7 +175,6 @@ fn main() {
         .file("curl/lib/parsedate.c")
         .file("curl/lib/progress.c")
         .file("curl/lib/rand.c")
-        .file("curl/lib/rename.c")
         .file("curl/lib/select.c")
         .file("curl/lib/sendf.c")
         .file("curl/lib/setopt.c")